A more thorough explanation:

Mental competence refers to a person’s ability to make rational decisions and express themselves. In the legal context, mental competency means a person’s capacity to understand the basic nature and purpose of court proceedings, their roles with respect to other parties in the courtroom, and the possible legal consequences of their actions.

For example, if someone is accused of a crime, they must be mentally competent to stand trial. This means they must understand the charges against them, the roles of the judge, jury, and lawyers, and the potential consequences of a guilty verdict.

In the United States, the law presumes that all adults are mentally competent to make decisions and be responsible for their actions. However, if someone challenges this presumption, they must prove that the person in question is more likely than not to be mentally incompetent. The party who claims the person is mentally incompetent may ask the court to order a mental competency evaluation carried out by a psychologist or psychiatrist.

It's important to note that the mere diagnosis of a mental or physical disorder may not be sufficient in itself to support a judicial determination that a person is mentally incompetent. The court usually takes into consideration a totality of circumstances, including the frequency, severity, and duration of a person’s mental handicap.